Effects of growth under UVA radiation on CO2 assimilation, carbon partitioning, PSII photochemistry and resistance to UVB radiation in Brassica napuscv. Topas

The effects of growth of Brassica napus L. cv. Topas under PAR or PAR+UVA r adiation was assessed with respect to sensitivity to subsequent exposure to UVB radiation. Despite the fact that growth under PAR+UVA induced minimal effects in photosystem II (PSII) photochemistry, growth under PAR+UVA inhib ited the accumulation of the photosynthetic end products, sucrose and starc h. This was associated with a decreased capacity for ribulose 1,5-bisphosph ate (RuBP) regeneration, a decreased capacity for light- and CO2-saturated rates of CO2 assimilation, a decrease in the apparent quantum yield for CO2 assimilation, an over-reduction of chloroplast stroma, an increased suscep tibility to the feedback effects on photosynthesis and a stimulation of gly colysis compared to controls grown under PAR. Subsequent exposure to UVB de creased the maximum Rubisco activity in leaves of both PAR- and PAR+UVA-gro wn plants. However, the decrease in the capacity for CO2 assimilation in PA R- grown plants exposed to UVB did not appear to be associated with limitat ions at the level of PSII linear electron transport, but rather with a decr eased capacity for sucrose biosynthesis, limited triose-P utilization and a decreased capacity for RuBP regeneration. In contrast, the decreased capac ity for CO2 assimilation in PAR+UVA-grown plants exposed to UVB was associa ted with an inhibition of PSII photochemistry and a decreased supply of ATP . Thus, growth under UVA radiation appears to induce feedback-limited photo synthesis and does not enhance resistance of Brassica napus to UVB radiatio n.